The pointing device of the present invention will be described in terms of a pen; the device may take other forms such as a cursor or mouse, but selection of a pen for purposes of description will facilitate understanding the invention. Therefore, references herein to "pen" should be interpreted to include other forms of pointing devices. A predetermined frequency is generated within the pen and is transmitted to the tablet surface. The location of the pen is determined by sensing the radiated signal using grid conductors within the tablet in the manner described in U.S. Pat. Nos. 5,218,174 and 5,225,637 assigned to the assignee of the present invention.
In the embodiment chosen for illustration, a predetermined nominal frequency is utilized as the signal being radiated from the pen to the tablet; information relating to the pressure being exerted by the user on the pen as it is forced against the surface of the tablet is contained in the variation of the frequency of the signal being radiated. The frequency is thus varied over a predetermined range in an manner that is related to the force being applied on the pen stylus by the user. The force applied by the user results in limited motion of the pen stylus, the initial motion of which is utilized to actuate a pen down switch; this switch actuation may be used to provide a signal to be radiated by the pen to the tablet to inform the latter that the pen is in contact with the tablet surface. Additional force applied by the user is subsequently utilized as a means for varying the radiated frequency to provide a basis for the tablet system to determine the force being used by the user as the pen travels over the surface of the tablet.
Prior art systems have attempted to sense force changes being applied to the pen through the utilization of magnetically permeable slugs that move in relation to an inductor. This technique requires a relatively long axial pen point or stylus displacement thus yielding a "spongy" or imprecise feel to the pen. Such systems may also make it difficult to modulate the output signal at light pen pressures. Such systems typically induce changes in system permeability and use phase changes to represent pen pressure.
Other prior art systems use force sensing resistors activated by the pressure exerted by the user on the pen. Such systems are complicated by the fact that relatively complicated circuitry is required to measure resistance change which is used to vary the capacitance of a variable capacitor, and which in turn is used to implement a frequency change. Resistance change systems typically exhibit abrupt initial signal changes; that is, as predetermined activation force is reached, the system transition to frequency modulation is abrupt. In such systems, independent adjustment of characteristics of the variable resistances is not possible; further, it may not provide repeatability because of changes of the characteristics through time and use and the tendency of such resistors to exhibit memory.